Process for upgrading C4 olefinic cuts

ABSTRACT

Process for upgrading an olefinic C 4  cut issued from a cracking or steam cracking unit, comprising subjecting said cut to isomerization conditions so as to convert at least 80% of its 1-butene content to 2-butenes, subjecting the so-treated C 4  cut to polymerization conditions so as to convert at least 90% of its isobutene content to isobutene dimers and trimers without substantially converting the normal butenes, separating the isobutene dimers and trimers, alkylating the remaining fraction and fractionating the latter to an alkylate, L P G and an isobutane containing fraction.

BACKGROUND OF THE INVENTION

The development of various processes of steam cracking and catalyticcracking of naphthas and gas-oils results in the supply on the market ofa C₄ cut which, after butadiene extraction, essentially contains amixture of butenes, isobutene, butane and isobutane which it isdesirable to upgrade.

The olefins, butenes and isobutene can be used for the synthesis of moreelaborated petrochemical products (alcohols, aldehydes, acids, nitriles,etc.), but the available amounts of these C₄ cuts are generally toolarge or at least may become too large for having recourse only to thisway to upgrade the C₄ cuts.

A second way consists of recycling said C₄ cut to the cracking unit(steam cracking) or to the catalytic cracking unit, after hydrogenationof the olefins; but, in this case, the isobutane contained in thehydrogenated cut, which amounts to more than 50% by weight of said cut,does not give sufficient ethylene yields and also results in asubstantial production of methane which practically can be used only asfuel.

A third way to upgrade said C₄ cut consists, after hydrogenationthereof, of separating the n-butane from the isobutane by distillationand recycling to the steam-cracking unit only the n-butane, whilerecovering the isobutane for more advantageous uses, such for example asthe production, by alkylation, of isoparaffinic gasolines. However, theyields of ethylene and propylene in a steam-cracking unit conducted inthis manner, do not exceed respectively 38 and 20% by weight and themethane production amounts to about 25% by weight.

A fourth way of upgrading the C₄ cut consists of alkylating the olefinsof said cut with the isobutane contained therein, so as to obtain themaximum yield of gasoline. However, in said alkylation step, thebehaviour of isobutene is less favorable than that of the butenes forobtaining gasoline of high octane number; as a matter of fact, theResearch (or Clear) octane numbers of the normal butene alkylates arehigher than those of the isobutene alkylates.

In order to improve this fourth way, it is possible, in a first stage,to subject the C₄ cut to polymerization during which an effort is madeto limit the total conversion of the normal butenes contained in the cutto less than 10% and during which at least 90% of the isobutene isconverted (preferably, at least 92% of the isobutene is converted); thishydrocarbon is converted essentially to isobutene dimers and trimers;subsequently, the resulting product is subjected to a fractionation soas to obtain, on the one hand, a first fraction which is fed to thealkylation unit and, on the other hand, a second fraction which issupplied to the gasoline pool, after partial or total hydrogenationthereof. The first fraction, fed to the alkylation stage contains inmajor part butane and isobutane and the butenes which have not reactedduring the polymerization. Such a process is described in the Britishpatent specification No. 2,017,746.

DETAILED DISCUSSION

The present invention is an improvement to the process disclosed in theBritish patent specification No. 2,017,746 for upgrading the C₄ cut, andconsists of subjecting the C₄ olefinic cut, not to a mere polymerizationreaction as above-described, but to a so-called "isomerizingpolymerization", which reaction provides not only for a selectiveconversion of isobutene to C₈ dimers and C₁₂ trimers, but also for thedisplacement of the double bond of 1-butene, whereby the major part of1-butene is converted to 2-butenes, thereby obtaining, under theselected operating conditions and catalysts, a composition close to thatcorresponding to the thermo-dynamic equilibrium. The other constituentsof the charge are not substantially converted during the isomerizationreaction. This conversion of 1-butene to 2-butenes is of primeimportance for the alkylation reaction performed after the fractionationof the polymerization effluent, since the products obtained byalkylation of 2-butenes have an octane number clearly higher than thatof the alkylates issued from 1-butene (the alkylates obtained from2-butenes have a R.O.N. from 5 to 7 points higher than that obtainedfrom 1-butene).

The operating conditions are such that the normal butenes contained inthe fraction fed to the alkylation zone contain by weight at least 85%of 2-butene and that, during the isomerization, at least 80% of the1-butene is converted to 2-butenes.

The process according to the invention is diagrammatically illustratedby the accompanying drawing. By the process of the invention, it ispossible to obtain a polymerizate which does not require any partial ortotal hydrogenation, in view of the high quality of said polymerizate.

The hydrogen feeding duct is not shown.

The olefinic C₄ cut is fed, through line 1, to the hydroisomerizationzone 2 where the isomerization of the double bond of the normal butenesis effected. This hydro-isomerization is performed in the presence of acatalyst used as a fixed bed or a moving bed or a fluidized bed, at atemperature from about 0° to 250° C., under a pressure from about 0.1 to20 MPa and at a liquid hydrocarbon flow rate (space velocity) of about0.2 to 20 volumes of hydrocarbon per volume of catatlyst and per hour.The catalyst contains generally at least one metal, preferably fromgroup VIII of the periodic classification of elements (for examplecobalt, nickel, palladium, etc.) deposited on a carrier of low acidity,for example transition alumina, silica, etc . . . having a specificsurface from about 20 to 300 m² per gram and a pore volume from about0.20 to 0.80 cc per gram.

The acidity of the carrier may be determined by the known test ofammonia adsorption, described in "Journal of Catalysis, 2, 211-222(1963): this method consists of heating the carrier to 600° C. undervacuum (i.e. under a pressure lower than about 1 Pa) up to a completegas removal; then, the carrier is placed in a calorimeter at 320° C. andammonia is introduced in such an amount that the final pressure of thesystem at equilibrium attains 40 k Pa and the evolved heat is thenmeasured.

The selected carriers have a neutralization heat, by ammonia adsorption,of less than 10 calories per gram at 320° C. under a pressure of 40 k Paand preferably less than 7 calories per gram. The neutralization heat ofthe final catalyst is substantially identical, i.e. less than 10calories per gram and, preferably, less than 7 calories per gram.

The catalyst may optionally operate in sulfided medium (for inhibitingthe hydrogenation properties of the metal), but this is not obligatory.In order to avoid a loss in the catalytic properties of the solid andalso to avoid secondary reactions, it is preferable to conduct thereaction under a hydrogen partial pressure, the hydrogen beingintroduced with the charge. The ratio hydrogen/hydrocarbon is thengenerally from 0.01 to 2 (ratio expressed in moles per mole).

At the outlet of the hydro-isomerization zone 2, the totality of theeffluent enriched with 2-butene (>85% of 2-butene with respect to thetotal contact of normal butenes in said effluent) is fed to apolymerization zone 3 which may be located either in a second reactor,after the isomerization reactor 2, or in the reactor already used forthe isomerization, the separate isomerization and polymerizationcatalyst beds being then generally superposed.

It must be observed that the effluent from the hydro-isomerization zonecontains a hydrogen excess which has not been used during theisomerization and said hydrogen excess also passes through thepolymerization zone without producing any disturbance of saidpolymerization reaction.

In the polymerization zone, the conditions are such that the isobutenereacts up to conversion rates higher than 90% by weight while the totalconversions of normal butenes (1-butene and cis-and trans-2-butenes)remain lower than or equal to 10% by weight and preferably lower than7%.

The polymerization reactions are generally performed in the presence ofa catalyst used, for example, in a fixed bed, at a temperature of fromabout 30° to 400° C., under a pressure from about 0.1 to 20 MPa (1 to200 bars), (preferably the temperature is from about 80° to 150° C. andthe pressure from 2 to 6 MPa) at a liquid hydrocarbon flow rate (spacevelocity) of from about 0.05 to 5 volumes per volume of catalyst and perhour.

The acid type catalyst may be a silica-alumina or a boron alumina or aborated alumina. It is also possible to select a catalyst obtained bytreatment of a transition alumina with an acid fluorine derivative,optionally with the addition of a silicic ester. According to thepresent invention, the catalyst used in the polymerization reactionexhibits qualities superior to those of other polymerization catalystssuch as phosphoric acid on Kieselguhr or on silica, or on quartz, orsuch catalysts as those of the "solid phosphoric acid" type, i.e.catalysts consisting of a siliceous material of high adsorpting power,impregnated with a high proportion of phosphoric acid, or catalysts suchas alumina and thoria gel mixtures, either co-precipitated or not, withoptional additions of chromium oxide, zinc oxide or oxide of anequivalent metal.

Preferably, according to the invention, there is used silica-aluminawhose silica content is from 60 to 95% by weight and preferably from 70to 90%, and preferably containing as additive from 0.1 to 5% by weightof zinc oxide. In view of the high exothermicity of the conversion, itis preferable that the isobutene content of the charge be lower thanabout 35% by weight, since otherwise the charge must be diluted forexample with butene or isobutane, and/or, for example, with a portion orthe totality of the butane and/or the isobutane issuing through line 17from the effluent of the hereunder-defined alkylation zone 11. Thisrecycled fraction of butane and/or isobutane is fed either to thepolymerization zone 3, when the polymerization reactor is independentfrom the isomerization reactor 2, or (as shown in the figure) at theinlet of the combined reactor. In the case of a combined reactor withtwo different beds, one for hydro-isomerization and the other forpolymerization, the recycled portion may be supplied directly to theinlet of said second catalyst bed so as to avoid too high a temperatureincrease.

Preferably, the temperatures, pressures and VVH are substantially thesame in the hydro-isomerization and polymerization reaction zones. Atthe outlet of the polymerization zone, the totality of the reactionmixture i.e. unconverted butenes, unconverted isobutene, isobutenedimers and trimers, butane and dilution butane, isobutane, etc., areconveyed through line 4 to a fractionation zone 5 wherefrom there arewithdrawn mainly, through line 10, a first fraction containing a majorpart of the butane, the isobutane, the isobutene and the butenes andthrough line 6, a second fraction mainly containing isobutene dimers andtrimers. This second fraction is fed to a second fractionation zone 7,wherefrom is withdrawn, at the top, through line 8, a mixture or "poly"gasoline containing a major portion of isobutene dimers and trimers (30to 70% by weight of dimers and 70 to 30% by weight of trimers, theproportions being expressed in percent of said mixture) which is fed,without requiring hydrogenation, to the gasoline pool, and at thebottom, through line 9, a residue having an initial boiling point higherthan 200° C. which can be fed to a fuel oil pool. The fraction withdrawnthrough line 10 from the fractionation zone 5 is fed to an alkylationzone 11.

Generally, the alkylation reaction is performed either in the presenceof a dissolved catalyst, i.e. in liquid phase, or in the presence of asolid catalyst, preferably used as a fixed bed, at a temperature from-20° to 200° C. and under a pressure from 10 kPa to 20 MPa. It is thuspossible to proceed in the liquid phase in the presence of a stronginorganic acid such as hydrofluoric acid or sulfuric acid, with orwithout addition of a Lewis acid such as boron trifluoride or antimonypentafluoride or alternatively, aluminum trichloride, and/or in theoptional presence of a Broonsted acid. It is even possible to proceed inthe vapor phase in the presence of a solid catalyst of the type of thephosphates, arsenates or stannates of polyvalent metals with borontrifluoride added thereto. Alkylation processes are also known, whichare performed in the presence of catalysts having a zeolite structure,with molecular sieves, in the presence of absence of silica-aluminas,for example with the optional presence of at least one metal such asnickel, palladium, rhodium, platinum, etc.

More particularly, the alkylation reaction may be performed attemperatures close to room temperature and under moderate pressures.

An additional isobutane amount can be added to the alkylation zone 11through line 16. This additional amount is preferable in order toobtain, at the inlet of the alkylation zone, a suitable molar ratioisobutane/olefins in the range from 6/1 to 10/1, this ratio beingselected to obtain an alkylate of optimum octane number. The isobutaneof line 16 comes from line 17 and/or 19.

There is no obtained, during the alkylation, an alkylate which iswithdrawn from line 12 and which can be fractionated in zone 13 in orderto obtain:

(a) LPG, which are withdrawn through line 14, usually containingsaturated hydrocarbons (iso and normal paraffins) having 4 carbon atomsper molecule, i.e. butanes of high isobutane content, which can be fedto the gasoline pool.

(b) An optional fraction (line 17) of high isobutane content, withdrawnfrom the top of the fractionation zone 13 and which is fed, according tothe needs, to the polymerization zone 3, through line 18 (on the figureline 18 is connected to the inlet of the isomerization zone 2 asabove-mentioned) so as to avoid therein too high temperature increasesand/or to the alkylation zone 11, through line 16 and,

(c) An alkylate which can be used, for example, as motor fuel, since thealkylation products generally have a clear octane number from 88 to 95.This alkylate is recovered through line 15.

Additional isobutane (necessary for diluting the charge and for dilutingthe cut fed to the alkylation zone) may be introduced through line 19.

According to a preferred embodiment of the invention, the alkylate ofline 15 and the "poly" gasoline of line 8 are collected together.

EXAMPLE 1

This example relates to the treatment of an olefinic C₄ cut from steamcracking; the charge composition is given in Table I.

                  TABLE I                                                         ______________________________________                                        CHARGE COMPOSITION (% BY WEIGHT)                                              ______________________________________                                               Isobutane                                                                               2                                                                   N-butane 10                                                                   Isobutene                                                                              46                                                                   1-butene 24                                                                   2-butenes                                                                              18                                                            ______________________________________                                    

This charge is treated in an isomerizing polymerization zone consistingof two successive catalyst fixed beds (2 and 3 in the figure); in thefirst bed, where the hydro-isomerization takes place, the catalyst isthat commercialized under reference LD 265 by Protacalyse Corporation;this catalyst is made of alumina of high purity containing 0.3% byweight of palladium. Its specific surface is 60 m² /g, its total porevolume is 0.50 cc/g, its filling density is 0.7 g/cc. It has the form ofballs of a diameter of 2 to 4 mm. The neutralization heat of thiscatalyst, measured under the above-mentioned conditions, is 6 caloriesper gram.

In the second bed, where the polymerization takes place, the catalyst isa silica-alumina sold in the trade as being of the Durabead PerlCatalysator Neu type manufactured by Kalichemie Corporation, containing0.2% by weight of zinc.

The operating conditions, in each of the two isomerization andpolymerization zones, are as follows:

    ______________________________________                                        VVH(in h.sup.-1) 2                                                            T °C.     110                                                          Pressure MPa     4                                                            ______________________________________                                    

In the hydro-isomerization zone, the ratio H₂ /HC is 0.5.

The Table II below reports the composition of the outlet effluent fromthe isomerization zone as well as the effluent composition at the outletof the polymerization zone. In the isomerization zone, 82.5% by weightof 1-butene have been converted to 2-butenes.

                  TABLE II                                                        ______________________________________                                                 ISOMERIZATION                                                                             POLYMERIZATION                                                    EFFLUENT    EFFLUENT                                                          % b.w.      % b.w.                                                   ______________________________________                                        Isobutane  2             2                                                    N-butane   10            10                                                   Isobutene  46            4.5                                                  1-butene   4.2           3.8                                                  2-butenes  37.8          34                                                   C.sub.8 -C.sub.12 cuts                                                                   --            42.1                                                 (poly)                                                                        Fuel-oil   --            3.6                                                  ______________________________________                                    

After fractionation of the polymerization effluent in zone 5 of thefigure, there is recovered, through line 10, normal butane, isobutane,isobutene and 1 and 2-butenes. This fraction is fed to the alkylationzone 11. However, in this cut, the isobutane proportion is insufficient(3.7% of isobutane for 80% of olefins) to obtain a molar ratioisobutane/olefins of at least 6, which is the minimum value required toavoid secondary reactions. Through line 16, there is added an additionalamount of isobutane corresponding to 6.2 times the weight of the chargeintroduced through line 10. The molar ratio isobutane/olefins is thenequal to 8.

The alkylation reaction is conducted in the presence of hydrofluoricacid, in reactor 11, stirred and cooled in such a manner as to maintainthe temperature of the reaction mixture at 30° C. The other operatingconditions are as follows:

pressure: 1.5 MPa

ratio isobutane/olefins: 8 (molar)

hydrofluoric acid volume (at 85% by weight) per hour and per volume ofolefin: 2

ratio by volume of acid to hydrocarbons: 1

After decantation, separation, washing and distillation of the reactioneffluent, there is obtained (in proportion to the charge fed throughline 10):

(1) through line 15: 83.8% by weight (with respect to the initialolefinic C₄ cut of line 1) of gasoline alkylate;

(2) through line 14: 10% by weight of LPG containing unreacted butane;

(3) through line 17; the isobutane excess which is recycled, partly tothe alkylation zone 11 through line 16, and partly to the polymerizationzone 3 through line 18 which, in the embodiment shown in the figure, isconnected, as explained above, to the inlet of the isomerization zoneand is used to dilute the fresh charge, since the latter contains morethan 35% of isobutene as above explained. There is so added to the freshcharge 44.2% by weight of isobutane (dilution isobutane). For sake ofsimplification of Table II, the dilution isobutane has not been takeninto account in the reported results.

Since the charge to be treated in the present example has aninsufficient isobutane content, it is necessary to use higher amounts ofisobutane by adding isobutane (amounting to 78% by weight with respectto the charge in line 1) through line 19.

The balance of the process with respect to the 100% of the charge andthe 44.2% of dilution isobutane, but without taking into account theadditional isobutane required in the alkylation reaction (additionalamount fed through line 16) is as follows by weight:

    ______________________________________                                        LPG                     10%                                                   Alkylate (line 15)      83.8%                                                 "poly" gasoline         42.1%                                                 Fuel-oil                3.6%                                                  Products from line 17   4.7%                                                  ______________________________________                                    

The obtained gasolines have the following octane numbers:

    ______________________________________                                                         Ethylated         Ethylated                                             RON   RON       MON     MON                                        ______________________________________                                        Alkylate     95      108.5     91.5  106.6                                    "Poly" gasoline                                                                            102     105.5     85    88                                       Alkylate and "poly"                                                                        97.6    107.9     89.6  100.7                                    gasoline mixture                                                              ______________________________________                                    

The octane number of the mixture shows a "synergistic" effect betweenthe components of the mixture, since the theroretical octane numbers areas follows: ##EQU1##

It is thus preferable to recover together the alkylate and the "poly"gasoline.

EXAMPLE 2

This example concerns the treatment of an olefinic C₄ cut from catalyticcracking in the same equipment, under the same conditions, and with thesame catalysts as in the preceding example.

The composition in % by weight of said C₄ cut is as follows:

    ______________________________________                                                isobutane      35                                                             n-butane       12                                                             isobutene      16                                                             1-butene       10                                                             2-butenes      27                                                     ______________________________________                                    

In the following Table III, the composition of the effluent at theoutlet of the hydro-isomerization zone is indicated as well as thecomposition of the effluent at the outlet of the polymerization zone.

                  TABLE III                                                       ______________________________________                                                 ISOMERIZATION                                                                             POLYMERIZATION                                                    EFFLUENT    EFFLUENT                                                          % b.w.      % b.w.                                                   ______________________________________                                        isobutene  35            35                                                   n-butane   12            12                                                   isobutene  16            1.5                                                  1-butene   3.7           3.3                                                  2-butenes  33.3          30                                                   "poly" gasoline                                                               (C.sub.8 -C.sub.12)                                                                      --            16.8                                                 fuel-oil   --            1.4                                                  ______________________________________                                    

After distillation of the polymerization effluent, there is recovered(a) a "poly" gasoline fraction, (b) a fuel oil fraction and (c) at thetop, the olefinic C₄ fraction enriched with 2-butenes, which isalkylated by means of isobutane with a ratio iso C₄ /olefin of 8,obtained by the addition of isobutane, but, in the present example, thisisobutane addition is only required for the starting period since thecut contains about enough of isobutane to satisfy the requirement of thealkylation reaction (35% by weight of isobutane for 34.9% of olefin).The other operating conditions are the same as in example 1.

The balance of the process with respect to the 100% of the charge andthe 2.8% of dilution isobutane added to the fresh charge through line18, but without taking into account the additional isobutane requiredfor the alkylation reaction, is as follows by weight:

    ______________________________________                                        LPG                     12%                                                   Alkylate                68.7%                                                 "poly" gasoline         16.7%                                                 Fuel oil                1.4%                                                  Product from line 17    4.1%                                                  ______________________________________                                    

The obtained gasolines have the following octane numbers:

    ______________________________________                                                         Ethylated         Ethylated                                             RON   RON       MON     MON                                        ______________________________________                                        Alkylate     95      108.5     91.5  106.5                                    "Poly" gasoline                                                                            102     105.5     85    88                                       Alkylate and "poly"                                                                        96.6    108       90.5  103                                      gasoline mixture                                                              ______________________________________                                    

EXAMPLE 3 (comparative)

The charge of example 1 is treated without making use of thehydro-isomerization zone 2. In other words, the charge passes directlyto zone 3 without addition of the hydrogen required for thehydro-isomerization. The other operating conditions are unchanged.

The balance of the process with respect to the totality of the charge(100%) and the 44.2% of dilution isobutane, is given below. The resultsof example 1 are given in parentheses:

    ______________________________________                                        LPG               10         (10)                                             Alkylate          80.7       (83.8)                                           "Poly" gasoline   42.1       (42.1)                                           Fuel oil          3.7        (3.6)                                            Total products    136.5      (139.5)                                          Losses            7.7        (4.7)                                            ______________________________________                                    

Octane number of the obtained product (into brackets, the octane numbersaccording to example 1):

    ______________________________________                                                     Ethylated            Ethylated                                   RON          RON        MON       MON                                         ______________________________________                                        Alkylate                                                                             92.5 (95) 105.5 (108.5)                                                                            89.5 (91.5)                                                                           104.5 (106.5)                             "Poly"                                                                        gasoline                                                                             102 (102) 105.5 (105.5)                                                                            85 (85) 88 (88)                                   Alkylate                                                                      and                                                                           "poly"                                                                        gasoline                                                                      mixture                                                                              95.5 (97.6)                                                                             105.5 (107.9)                                                                            88 (89.6)                                                                             99 (100.7)                                ______________________________________                                    

It is observed that the octane numbers of the mixtures are not so goodas the calculated theoretical octane numbers of example 1. Furthermorein the present case, it would be more convenient to hydrogenate the"poly" gasoline in order to improve its qualities.

EXAMPLE 4 (comparative)

The charge of example 2 is treated without making use of thehydro-isomerization zone 2. In other words, the charge passes directlyto zone 3. The other operating conditions remain unchanged. The balanceof the process with respect to the totality of the charge and the 2.9%of dilution isobutane is given below: (in parentheses the results ofexample 2)

    ______________________________________                                        LPG               12         (12)                                             Alkylate          67         (68.7)                                           "Poly" gasoline   17.1       (16.7)                                           Fuel-oil          1.5        (1.4)                                            Total product     97.6       (98.8)                                           Losses            5.3        (4.1)                                            ______________________________________                                    

The obtained gasolines, useful as motor fuel, have the following octanenumbers (into parentheses, the octane numbers obtained in example 2):

    ______________________________________                                               RON     Ethylated           Ethylated                                         RON     RON        MON      MON                                        ______________________________________                                        Alkylate 94 (95)   107.5 (108.5)                                                                            91 (91.5)                                                                            106 (106.5)                              "Poly"   102 (102) 105.5 (105.5)                                                                            85 (85)                                                                              88 (88)                                  gasoline                                                                      Alkylated                                                                     and                                                                           "poly"                                                                        gasoline                                                                      mixture  95.5 (96.5)                                                                             107 (108)  90 (90.5)                                                                            102.5 (103)                              ______________________________________                                    

The advantage of the process according to the present invention clearlyappears from the comparison of the 4 examples. Not only better yields ofgasoline, useful as motor fuel, are obtained, but also the obtainedgasolines are of higher quality.

What is claimed is:
 1. A process for upgrading an olefinic C₄ hydrocarbon cracking or steam cracking cut, comprising the steps of:(a) feeding the olefinic cut to a hydro-isomerization zone where the cut is treated in the presence of hydrogen and an isomerization catalyst containing at least one group VIII metal, deposited on a carrier, under such conditions that at least 80% of the 1-butene in said cut is isomerized to 2-butenes, the percentages of the other components of the cut being essentially unchanged, and such that, at the end of the isomerization reaction, the normal butenes contained in the hydro-isomerization effluent comprise at least 85% by weight of 2-butenes and less than 15% by weight of 1-butene; (b) feeding the entire effluent from the hydro-isomerization zone, without any intermediary fractionation, to a catalytic polymerization zone containing a catalyst different from the hydro-isomerization catalyst, the polymerization catalyst being a fluorinated alumina, a boron alumina, or a silica-alumina, and converting at least 90% of the isobutene contained in the hydroisomerization effluent mainly to isobutene dimers and trimers, the aggregate conversion of the normal butenes contained in the cut being kept lower than or at most equal to 10% by weight, the butene and isobutane contained in the effluent of the hydro-isomerization zone being substantially unconverted; (c) fractionating the effluent from the polymerization zone in a fractionation zone and recovering therefrom a first fraction comprising in major part isobutene dimers and trimers, and a second fraction comprising in major part isobutane, butane and butenes; and feeding said second fraction to an alkylation zone, fractionating the alkylation effluent in a fractionation zone, and recovering therefrom (i) an alkylate, (ii) LPG having a high content of saturated C₄ hydrocarbons, and (iii) a fraction comprising mainly isobutane.
 2. A process according to claim 1, wherein in step (d), the alkylate (i) is combined with the first fraction of isobutene dimers and trimers mixture recovered in step (c), to produce a blended gasoline.
 3. A process according to claim 1, wherein the catalyst in the hydro-isomerization zone has a neutralization heat, by ammonia adsorption, lower than 10 calories per gram at 320° C. under a pressure of 40 kPa.
 4. A process according to claim 3, wherein the catalyst carrier of the hydro-isomerization zone is alumina.
 5. A process according to claim 1, wherein the olefinic C₄ cut contains at least about 35% by weight of isobutene, and the cut is diluted with an amount of isobutene sufficient to reduce the proportion of isobutene to below 35% by weight.
 6. The process according to claim 1, wherein in step (a), the group VIII metal of the hydro-isomerization catalyst is cobalt, nickel or palladium.
 7. A process according to claim 6, wherein the metal is palladium.
 8. A process according to claim 1, wherein the hydro-isomerization reaction of step (a) is conducted at a temperature of 0°-250° C., under a pressure of about 0.1-20 MPa, with a liquid hydrocarbons flow rate of about 0.2-20 volumes of hydrocarbons per volume of catalyst and per hour; and wherein the polymerization reaction of step (b) is conducted at a temperature of 30°-400° C., under a pressure of about 0.1-20 MPa, with a liquid hydrocarbons flow rate of about 0.05-5 volumes per volume of catalyst and per hour.
 9. A process according to claim 8, wherein the polymerization reaction is conducted at 80°-150° C., under a pressure of 2-6 MPa.
 10. A process according to claim 8, wherein the temperature, pressure and hydrocarbons flow rate conditions are substantially the same for the hydro-isomerization as for the polymerization.
 11. A process according to claim 5, wherein the isobutane added to dilute the charge is at least a portion of the fraction (iii) comprising mainly isobutane recovered in step (d).
 12. A process according to claim 1, wherein in step (d), said second fraction fed to the alkylation zone is diluted with isobutane in an amount sufficient to achieve a molar ratio of isobutane to olefins of 6:1-10:1.
 13. A process according to claim 12, wherein the isobutane diluent comprises at least a portion of the fraction (iii) comprising mainly isobutane recovered in step (d). 